Molecular Beam Epitaxy (MBE) systems employ super high vacuum chambers equipped with a means to grow layers on a substrate by evaporation of source materials and controllable means to direct beams of such source materials toward said substrate.
Several ways to measure beam fluxes have also been reported in connection with MBE. These include pressure gauges, such as ionization gauges, placed in the beam path. These gauges respond to different materials differently, an effect which cannot be eliminated and they are difficult to compensate.
Optical techniques including reflection high energy electron diffraction, RHEED, atomic absorption and resonance fluorescence have also been employed. RHEED only gives meaningful data when monitoring growth which naturally grows layer by layer. This technique is inapplicable to many complex crystal structures.
Other MBE apparatus have traditionally employed crystal rate monitors to measure flux from the source. Most such monitor devices were unable to provide an accuracy greater than 5% and in general they were unable to be placed in the direct beam. These monitors could not differentiate between atomic species and do not have sufficient sensitivity for modern sequential deposition methods.
Atomic absorption (AA) has been used earlier in conjunction with MBE as disclosed in Japanese application No. 63-77799, filed Mar. 30, 1988. However, in prior art atomic absorption (AA) systems, the systems have not properly compensated for all the sources of error, such as changes in absorbance due to window coating, variations in the AA source lamp current, or in the circuit drifts or absorption due to other materials. AA spectroscopy is a known technique for making precise, very low density measurements. Many error correcting techniques and background correction techniques have been employed in AA spectroscopy. Each such prior art correction technique has generally depended on the fact that in traditional AA that the line width of the probing beams is at least 3-10 times narrower than the line width of the atoms in the sample cloud. In prior art AA, the sample cloud atoms are typically at 1 atmosphere pressure at elevated temperature, i.e. T&gt;1000.degree. C. In an MBE, the pressure range is typically much lower, and can be low as 10.sup.-10 -10.sup.-11 Torr and the line width of the atom spectra are of the same order but narrower than the line width from the hollow cathode lamp probe beam.
Atomic absorption techniques as part of process control in sputtering is also known as shown in U.S. Pat. No. 5,089,104. In the '104 patent, the concentration of sputtered particles from the is measured via atomic absorption and compared to calibration ratio data. No background absorption error or electrical drift error compensation is indicated and the only feedback control is to maintain the power level to the ion beam sources to provide the calibration ratio values. Calibration techniques in atomic absorption spectroscopy measurements involving dual beams is also well known. Compensation for the absorption from interfering materials and for the electronic drifts of the source lamp are corrected by log ratio comparison of the received transmitted intensity of a probe beam signal in the two paths. This double beam technique introduces an error because the optical components of the two paths are not identical because of different elements such as lens or window coatings and because different electrical detectors may be involved, each of which also introduce errors.
There are also several known single beam background AA correction techniques. It is known to modulate with two different cathode source lamp currents, where one is an overdriven hollow cathode lamp current such as described in U.S. Pat. No. 4,462,685. That scheme provides a different lamp intensity at the absorber wavelength for the two values of lamp current. So long as complete characterization of the spectral output of the lamps are available, ratio comparison of the two signals can correct for background induced errors. However, this technique does not work for all elements and causes instabilities in other lamp sources. Another single beam approach employs the so-called Zeeman effect as described in U.S. Pat. Re. No. 32022. This Zeeman system is a relatively complex and expensive system requiring an alternating magnetic field applied to the sample for best results.
There is a need for a long term stable, simple, reliable and precise, background corrected and normalized single beam technique which doesn't require magnetic splitting of the vapors or overdriving of the source lamps in order to measure the flux from thermal sources in an MBE system.